The present invention generally relates to interference suppression, and particularly relates to demodulating a desired signal while suppressing an interfering signal having a different modulation.
Some wireless communication systems employ several modulation schemes for transmitting information. The modulation scheme used to communicate information over a particular channel depends on several factors such as the degree of channel interference and noise. Generally, a higher-capacity modulation scheme is used to convey information when noise and interference levels are relatively low. For example, Enhanced Data rates for GSM Evolution (EDGE) based systems utilize a modulation scheme such as 8PSK (or possibly an even higher-order modulation scheme such as 16-QAM or 32-QAM) when noise and interference levels are sufficiently low. However, GMSK is used to convey information when noise and interference levels are high. EDGE-based systems transmit information at the same symbol rate regardless of modulation type, but 8PSK provides a 3× data density improvement over GMSK because 8PSK encodes three bits of information per symbol compared to one bit for GMSK.
The GMSK modulation transmits one bit per symbol interval. In a commonly used model for the GMSK signal in GSM/EDGE, the bit is modulated using +1 or −1 (or phases of 0 and 180 degrees), but the phase of the signal is progressively shifted by pi/2 for each symbol interval. In a similar fashion, the 8PSK modulation in GSM/EDGE uses one of eight phases (0, pi/4, pi/2, 3pi/4 and their negative counterparts) to encode three bits of information, while progressively applying a phase shift of 3pi/8 per symbol interval. The phase shift applied to each signal helps shape the signal characteristics to be more suitable for amplification purposes, whereas the different phase shifts applied to GMSK and 8PSK help in distinguishing the two signals for the purposes of blindly detecting the modulation used in a particular frame.
While 8PSK modulation provides increased data density, GMSK modulated signals are less susceptible to error in high noise and interference environments because the phase and amplitude difference representing adjacent symbols is greater for GMSK than for 8PSK or QAM. Thus, EDGE communication systems tend to transmit information over highly disadvantaged channels using GMSK while 8PSK or QAM is used for less disadvantaged channels.
Mobile terminals operating in an EDGE or similar environment are subject to noise and co-channel interference, i.e., interference arising from other communication channels in the same or different cell. EDGE-based systems conventionally employ a Single Antenna Interference Cancellation (SAIC) technique for suppressing co-channel interference when both the interfering signal and desired signal are modulated according to GMSK. However, in many instances, the interfering signal does not have the same modulations as the desired signal. SAIC is not effective at cancelling co-channel interference when the desired and interfering signals have different modulations. For example, SAIC is ineffective at suppressing an interfering GMSK-modulated signal when the desired signal is modulated according to 8PSK. Other interference suppression techniques account for modulation differences, but require a highly complex equalizer and calculation of pre-filter coefficients separate from channel response estimation.